Career
At the outbreak of, Gronau was recalled from retirement to take command of the newly formed IV Reserve Corps as part of the 1st Army, which was on the right wing of the forces that invaded France and Belgium as part of the Schlieffen Plan offensive in August 1914. lieutenant played a significant part in the First Battle of the Marne. In September 1915, Gronau was appointed to command of XXXXI Reserve Corps (swapping places with Generalleutnant Arnold von Winckler).
The Corps was upgraded to form Armee-Gruppe Gronau on 20 September 1915.
Gronau remained in concurrent command of the Corps and the Armee-Gruppe. lieutenant served with the Army of the Bug throughout its existence.
On 18 September 1916 it was raised to the status of an Armee-Abteilung (Army Detachment). lieutenant remained on the Eastern Front until dissolved on 27 March 1918.
On 4 October 1916 he was awarded the Pour le Mérite for military bravery.
On 23 February 1890 Gronau married Luise Gerischer (20 July 1867 – 25 June 1926). The marriage produced three sons. Armee-Abteilung or Army Detachment in the sense of "something detached from an Army".
lieutenant is not under the command of an Army so is in itself a small Army.
Armee-Gruppe or Army Group in the sense of a group within an Army and under its command, generally formed as a temporary measure for a specific task. Heeresgruppe or Army Group in the sense of a number of armies under a single commander.